1. Field of the Invention
The present invention relates generally to oil recovery. More specifically, the invention relates to a means of recovering oil from fractured reservoirs.
2. Description of the Related Art
The current oil drilling and pumping technologies and processes have successfully recovered large quantities of oil from un-fractured underground reservoirs. However, current technology fails to recover oil, natural gas, and other fuels trapped in fractured or discontinuous reservoirs in an efficient manner. In certain instances, the conventional technology lacks methods to recover oil in certain reservoirs.
Particularly, conventional technology fails to recover oil in fractured carbonate reservoirs. Carbonate reservoirs are formed as sedimentary rocks deposited in static marine environments. The static formation of the rocks leads to diagenesis, or any chemical, physical, or biological change undergone by sediments after initial deposition, as well as during and after lithification. Fractured carbonate reservoirs typically result from severe diagenesis. The diagenetically altered sediment results in heterogeneous carbonate grain matrix within a rock formation or underground structure. Further, the porosity of the carbonate reservoir results in at least three morphological classifications including, connected porosity, fracture porosity, and vugs. These classifications result in a complex flow path for the oil in the reservoir, and inhibit efficient oil production. Other reservoir formations, such as stylolite structures form flow barriers within the reservoir, inhibiting recovery of the liquid hydrocarbons, such as oil.
The wettability of the carbonate reservoir structure may further interfere with efficient oil recovery in conventional techniques. The carbonate matrix comprising the reservoir may have increased adhesive properties with certain oils. The increased adhesion of oil is termed oil-wet or mixed-wet. The oil in these instances adheres more strongly to the geological formations in the reservoir and impedes recovery. The wettability of the carbonates determines the efficiency of water, and pressurized steam methods for recovery of oil in the reservoir. The wettability of may cause the fluids used in these techniques to flow through water-wet regions of the reservoir and thereby fail to recover oil that is adhering to the underground formations.
About eighty percent of carbonate reservoirs are oil-wet or mixed-wet, most of which are also fractured. Secondary recovery using conventional water flooding methods are not effective in recovering oil from the matrix because oil tends to adhere strongly to the reservoir matrix; it recovers oil from only the fractures. Other existing processes to improve recovery rates such as the injection of surfactant solutions into rock fractures are limited because surfactant solutions do not efficiently diffuse into the matrix, if the fracture spacing is large.
Consequently, there is a need in the industry for a process for oil recovery from oil-wet and mixed-wet fractured reservoirs, in order to achieve maximum oil production. Further, there is a need to enhance oil recovery in oil-wet and mixed-wet fractured reservoirs, particularly in cases where primary oil production has led to gas formation in the reservoir.